Customs law is almost as vague & contradictory as copyright law, either lacking definitions (such as what constitutes personal use) or with definitions that allow easy circumvention. Their approach is often to seize questionable goods & let you sue for recovery but, as with most law enforcement, whether or not to actually enforce a law is up to the individual officer. (I.e. if they don't want to enforce the law then they simply "don't notice" the violation.)